1. Technical Field of the Invention
The present invention relates generally to the telecommunications field and, in particular, to a method for combining signals on a digital interface between units.
2. Description of Related Art
In digital cellular communication systems, such as, for example, the Global System for Mobile Communications (GSM), distributed Radio Base Stations (RBSs) can be configured with Common Function (CF) and base band digital signal processing (BB DSP) components in one unit, and radio transmitter/receiver components in one or more separate radio units. Essentially, the radio units are location-dependent, because their coverage is determined by their respective antenna""s location and direction. On the other hand, the CF/BB DSP units are location-independent, and a single CF/BB DSP xe2x80x9cpoolxe2x80x9d unit can serve many different radio units, and thereby many cells. Typically, such radio units are multi-carrier radios.
Alternatively, a transceiver-oriented RBS configuration can be used, which has one CF unit and a plurality of separate transceivers (TRXs). In both of these RBS configurations, the CF part of the respective unit is responsible for switching between the transport network (e.g., Abis) and the TRXs or radio units. Also, the CF unit contains a Reference Oscillator, which is frequency-locked to the transport network (or another reference), and a Reference Clock which states the time. The carrier frequencies used by the RF components in the TRXs or radio units are derived from the Reference Oscillator, and the start times for the transmissions and receptions are based on the Reference Clock""s time.
In any event, a digital interface is required to interconnect the CF/BB DSP unit with the radio units (xe2x80x9cpoolxe2x80x9d system), or the CF unit with the TRXs (transceiver-oriented system). Typically, such a digital interface has to convey four independent pieces of information: asynchronous data; frequency reference information; absolute time information; and reset information.
Essentially, the asynchronous data is control information about the burst and actual data to be transmitted, which the BB DSP unit conveys to the other units. The frequency reference information can be generated, for example, by filtering out the frequency of the transmission interface signal. However, since the frequency reference is used to generate the radio units, or TRXs"" RF carrier, the accuracy requirements imposed on the frequency reference are very stringent. For example, the GSM Technical Specification states that no carrier may deviate more than 50 ppb from the Reference Oscillator frequency. The absolute time information is used to ensure that the radio transmission and BB DSP components have the same perception of time. Notably, in an RBS having multiple radio units, it is essential that the different radio units have the same perception of time. As such, the GSM Technical Specification states that the timing difference between any two transceivers/radio units must not be greater than 915 ns. Finally, the reset information being conveyed is also important, because it is essential to be able to xe2x80x9cresetxe2x80x9d the radio components if, for example, their software should lock-up for any reason.
A significant problem associated with the design of a digital interface between the CF/BB DSP unit and radio unit (or CF unit and TRXs) in a cellular RBS is how to minimize the number of wires (or optical conductors) between the units. There are a number of reasons why it is important to keep the number of interface wires (or optical conductors) to a minimum. For example, the electrical or optical connectors used at the units take up a significant amount of space. Also, the number of pin connections to/from integrated circuits (e.g., ASICs) at the inputs/outputs of the units are limited in number. Furthermore, for relatively long distances (e.g., a wire or fiber optic conductor between units can be hundreds of meters long), the cost of the transmission media can be quite high (especially for optic fibers). An advantage of reducing the number of conductors in a digital interface is that this reduction is typically accompanied by reduced power consumption in the RBS involved. Yet another advantage of keeping the number of conductors in a digital interface to a minimum is that the costs of the modems and transceivers involved are reduced.
Under normal operating conditions, separate interfaces are used to convey the asynchronous data, frequency reference, timing, and reset information. As such, a basic problem with imposing a design requirement that the digital interface between RBS units is to carry as few signals as possible is that, for the existing systems, the digital interface has to carry the frequency reference and time reference signals, in addition to the asynchronous data. Notably, as mentioned above, these reference signals have extremely stringent accuracy or resolution requirements imposed. Furthermore, there are no existing asynchronous digital interfaces that carry a frequency reference signal.
As mentioned above, the time reference is used by the RBS""s signal processing component to transmit bursts at the correct instant of time. In fact, for mobile positioning applications (i.e., determining the position of mobile terminals in a cellular network), the accuracy requirement imposed on the timing reference can be extremely stringent ( less than 100 ns variation).
The reset signal conveyed over the digital interface should be designed so that it can be received and interpreted by a simple analog circuit, which in turn, can output a signal to reset the RBS""s components as required (e.g., after a software lockup). In other words, if an Application Specific Integrated Circuit (ASIC) or other circuit is used primarily for re-initiating RBS unit operations after a software lockup, the simple analog circuit""s operation to reset the ASIC should be independent of the ASIC""s (or other circuit""s) operation. As such, the existing RBSs do not provide such a capability without additional conductors. However, as described in detail below, the present invention successfully resolves the above-described problems.
In accordance with the present invention, a method for combining signals being conveyed across a digital interface is provided. In one embodiment of the present invention, asynchronous data are superimposed on a subharmonic of a frequency. The subharmonic can be used to carry a frequency reference across the digital interface. The frequency reference can be used, for example, to generate a carrier frequency in a radio or transceiver unit. In a second embodiment of the present invention, a time stamp algorithm is used to carry a time reference from a master circuit to a slave circuit across a digital interface. The transmission delay between the master circuit and slave circuit is measured. The master circuit transmits time correction messages to the slave circuit, based on the time the correction message is transmitted plus the transmission delay. In a third embodiment of the present invention, a reset signal for resetting a radio transceiver unit is carried across a digital interface by omitting the transfer of the frequency reference from the first embodiment for a predetermined time. A lowpass filter or watchdog circuit can be used at the receiving side of the interface to recognize the absence of the frequency reference and, in turn, initiate a reset procedure at the receiving side.
An important technical advantage of the present invention is that the number of conductors in a digital interface can be minimized.
Another important technical advantage of the present invention is that by minimizing the number of conductors used in a digital interface, the power consumption of the equipment in use is minimized.
Yet another important technical advantage of the present invention is that by minimizing the number of conductors used in a digital interface, the cost of the equipment in use is minimized.
Still another important technical advantage of the present invention is that by minimizing the number of conductors used in a digital interface, the space requirements for the equipment in use are minimized.
Still another important technical advantage of the present invention is that by minimizing the number of conductors used in a digital interface, the number of integrated circuit pins required for the equipment in use is minimized.
Still another important technical advantage of the present invention is that a digital interface can be used to provide simpler integrated circuit implementation, carry asynchronous data and a frequency reference simultaneously on one conductor, convey a highly accurate time reference using one conductor, convey a modulation signal that is DC-free, and convey reset information, asynchronous data, and a frequency reference concurrently on one conductor.